The microhardness evaluation of materials allows a quick prediction of their mechanical properties and the identification of morphological changes, making it an excellent tool for the characterization of parts with microstructural variations over time or that have geometric variations. In this work, instrumented microindentation test is performed on poly(vinylidene fluoride) (PVDF) specimens with different degrees of crystallinity and crystalline structures. The microstructural differences are generated by changing the cooling temperature and the annealing duration. α‐phase and β‐phase are detected by FTIR analysis in all samples, which presented the same spectra. The mathematical relation between the microhardness and the crystallinity degree of PVDF is found, and the theoretical microhardness values of the crystalline and amorphous phases are determined. Microhardness is also related to the melting temperature of the polymer, which varies as a function of crystals perfection. This relationship is expressed through a linear equation.
The computational modeling of instrumented indentation tests used to characterize material properties is challenging. It is mainly due to the computational techniques demanded to couple the complex physical mechanisms involved, such as, for example, the time-dependent inelastic material response to loads during contact. Therefore, this work aims to simulate the mechanical response of the poly vinylidene fluoride (PVDF) during a micro-indentation test considering a viscoplastic material model, and a prescribed load approach, using the finite element method. Further, model validation is performed based on experimental data measured during the contact between the indenter and the PVDF. Numerical analyses were performed using COMSOL Multiphysics finite element software considering the loading scheme of the experimental tests of 800 mN/ min rate during loading and unloading, and a 400 mN constant load, held by 30 s. Finally, a viscoplastic Chaboche constitutive model is presented considering two cases: (1) a perfectly plastic behavior, and (2) a nonlinear isotropic hardening behavior based on Voce and Hockett-Sherby exponential laws. While the latter models exhibit some discrepancy in capturing the experimental behavior, the former one has shown excellent agreement with the load-depth curves obtained experimentally, achieving the best fitting for the set of Chaboche parameters: A = 1 s −1 , n = 4.62 and ref = 132 MPa. Moreover, several phenomenological features of viscoplastic behavior such as rate dependence, plastic flow (or creep) and stress relaxation were accurately provided by the Chaboche model when describing the behavior of the PVDF material. Keywords Viscoplasticity • Polymers • Microindentation • Finite elements Nomenclature E Young's modulus F y Yield function h max Maximum indentation depth h r Residual or final indentation depth h m , r m PVDF sample thickness and radius J 2 Second deviatoric stress invariant P Applied load P max Maximum applied load Q p Plastic potential r i Indenter radius S Deviatoric stress tensor t h Holding load time vpe Effective viscoplastic strain vp Viscoplastic strain tensor Cauchy's stress tensor Mises Effective von Mises stres ys Yield Stress ys0 Initial yield stress sat , Voce model parameters Technical Editor: João Marciano Laredo dos Reis.
Polymeric materials are widely used in structural components and systems, and the accurate prediction of their complex time‐dependent behavior is critical. Several constitutive models are available for different types of mechanical behaviors and loading conditions. However, selecting the model and the correct calibration of its parameters is often a challenge. This paper evaluates the applicability of the two‐layer viscoplasticity (TLV) model to predict the viscoplastic behavior of polyvinylidene fluoride (PVDF) in a large range of strain rates and complex states of strain. The setting of parameters and model validation was made by performing experimentally and numerically uniaxial compression tests and relaxation tests. The model parameters were also analyzed in performing microindentation tests. The TLV model using the same set of parameters selected well predicted the PVDF behavior in a wide variety of compressive loading conditions.
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